Chapter 5. Quantum chemistry in Molecular Modeling


5.9 Solvation

The effect of a solvent can be incorporated in quantum-chemical calculations most easily by considering it as a continuous dielectric medium, characterized by a dielectric constant. The electric field caused by the molecule induces a polarization of the medium, which in turn acts on the electrons in the molecule (Self-Consistent Reaction Field, SCRF)[7].
The model thus contains the quantum-mechanical description of the molecule and a classical medium. The problem is to choose where to locate the boundary between quantum system and medium. In the Gaussian programs a simple approximation is used in which the volume of the solute is used to compute the radius of a cavity which forms the hypothetical surface of the molecule. Spartan offers solvation models for the semi-empirical Hamiltonians.
This method, in which the molecular surface is constructed from atomic (Born) radii, requires parameters for each atom [6, 20]. In most cases, solvation hardly affects the structure of a molecule (relative to the gas phase), but in cases of polar molecules, zwitterions or ions, the relative energies can be changed dramatically [6].

References:

[6] Spartan User's Guide, version 3.0,
Wavefunction, Inc., 1993.
[7] Foresman, J.B.; Frisch, A.
Exploring Chemistry with Electronic Structure Methods: A Guide to Using Gaussian, Gaussian Inc., 1993.
[20] Cramer, C.J.; Truhlar, D.G.,
Science, 1992, 256, 213 - 217.

Next paragraph, 5.10 Properties derived from the wavefunction
Previous paragraph, 5.8 Quality of semi-empirical results
Chapter 5 MM Syllabus 1995 MODIFIED November 8, 1995
Fred Brouwer, Lab. of Organic Chemistry, University of Amsterdam.